1. Field of the Invention
The present invention relates to a light-emitting device having a light-emitting element in each pixel.
2. Description of the Related Art
A light-emitting element is highly visible since it emits light by itself, and has a feature that it does not need a backlight required for a liquid crystal display device (LCD), thereby being best suited for making the device thin, and besides, the viewing angle is wider than that of an LCD. Therefore, a light-emitting device using a light-emitting element draws attention as a display device that substitutes a CRT and an LCD and makes toward practical use. An OLED (Organic Light Emitting Diode), which is one of light-emitting elements, includes a layer containing an electroluminescent material that can obtain luminescence (Electroluminescence) by applying an electric field (hereinafter referred to as an electroluminescent layer), an anode, and a cathode. Luminescence can be obtained by combining a hole injected from the anode with an electron injected from the cathode in the electroluminescent layer.
Injection properties of a hole and an electron into an electroluminescent layer are assumed one index by a size of a work function of a material that forms an electrode. It is desirable that a material with a high work function is used for an electrode on the side a hole is injected (an anode), and a material with a low work function is used for an electrode on the side an electron is injected (a cathode). Specifically, indium tin oxide (ITO) of which work function is 5 eV is generally used for an anode.
As one mode of light-emitting devices that applied such a light-emitting element, a light-emitting device in which an electroluminescent layer is sandwiched between an electrode extending in one direction (parallel electrode) and an electrode extending in a direction intersecting with it (a column electrode) and then arranged in a matrix is known (see Reference 1: C. W. Tang, S. A. VanSlyke, and C. H. Chen, Journal of Applied Physics, vol. 65, p. 3610, 1989).
In the meantime, a backlight is not used for a light-emitting device; therefore, there is a high tendency that the total power consumption of the light-emitting device relies on a performance of a light-emitting element in each pixel. That is, low power consumption can be realized with high external quantum efficiency (the number of the photons externally extracted/the number of the injected carriers). The external quantum efficiency can be enhanced by improving the extraction efficiency (the number of the photons externally extracted/the number of the discharged photons).
However, a ratio of an area in which luminescence can be naturally obtained to an entire pixel portion (an aperture ratio) is decreased as the pixel is made to be higher precision. That is, it is considered that a relation between high precision and improvement of the extraction efficiency is tradeoff in some degree. As a result, it is difficult to increase the external quantum efficiency.